This invention relates generally to hydraulic braking circuits for automobile vehicles, which comprise a modulation device disposed between on the one hand a transmitter device such as a hydraulic master cylinder adapted to supply fluid under pressure under the control of an operating means, (which in practice is a brake pedal), and on the other hand the braking receiver such as a slave cylinder of a wheel brake, the modulation device being controlled by a pickup sensitive to the speed of rotation of the respective wheel and which comprises at least two valves, namely an isolation valve which when open is adapted to establish communication between the transmitter device and the braking receiver, and a pressure drop regulation valve which when open is adapted to establish communication between the braking receiver and a discharge.
A braking circuit of this kind is described in the French patent filed under No. 76 14030 and published under No. 2,350,992. As a matter of public information only, it is to be noted that U.S. Pat. No. 4,155,604 granted to Fenart on May 22, 1979, claims priority from the French Patent Application No. 76 14030.
During normal braking, that is to say braking which is sufficiently moderate for there to be no risk of the locking of a wheel, the isolating valve is open and the pressure drop regulation valve closed; there is direct communication through the isolating valve between the transmitter device and the braking receiver.
When in the course of braking a risk of locking arises, the pickup sensitive to the speed of the respective wheel comes into action and brings about on the one hand the closing of the isolating valve, thereby immediately interrupting all direct communication between the transmitter device and the braking receiver, and on the other hand the opening of the pressure drop regulation valve, thereby connecting the braking receiver in a regulated manner to the discharge.
When the risk of the locking of a wheel is eliminated on the discharge of the braking receiver in this manner, the pickup sensitive to the speed of the wheel ceases to act, thereby on the one hand bringing about the closing of the pressure drop regulation valve and on the other hand opening a third valve known as the pressure rise regulation valve, which was closed during the preceding phase and which, once opened, (and as long as the driver of the vehicle maintains his action on the brake pedal) brings about a regulated rise of pressure in the braking receiver, the isolating valve remaining at least temporarily closed.
If critical conditions liable to lead to the locking of a wheel should then occur again, the modulation device will once more intervene in accordance with a pressure drop and pressure rise cycle of the type briefly analysed above.
In practice, and where the drive of the vehicle continues to operate the brake pedal, a succession of such cycles will occur.
Each cycle corresponds to a variation of the pressure in the entire brake circuit, and particularly between the braking receiver and the transmitter device.
When this transmitter device is a proportioner, that is to say an assembly composed of a high pressure accumulator and a pump feeding the accumulator, the downstream pressure being modulated under the control of the brake pedal, as described more particularly in the previously mentioned French patent, such a variation of pressure does not give rise to any major mechanical inconvenience; however, when it is transmitted to the brake pedal it may constitute a nuisance to the driver of the vehicle.
When the transmitter device is a master cylinder, this nuisance is accentuated.
Furthermore, this master cylinder receives directly the variation of pressure due to the cycle of intervention of the modulation device, to the detriment of its working life.
The main object of the present invention is to reduce or eliminate these disadvantages.